In the manufacture of electronic circuits on ceramic substrates, circuit conductors and tantalum resistances and sometimes many other types of circuit components are formed on the surface of a ceramic substrate. Land areas at each end of a tantalum resistor or at each end of a conductor form the contact points on which separate electronic component leads are bonded to the circuit elements that have been formed on the surface of the substrate. These land areas are sometimes solder plated but are preferably gold plated. Solder and flux are then placed on the land areas. The substrates carrying the separate components that are as yet not solder-fused to the land areas are then heated to reflow or fuse the solder at the land areas and thus bond the separate components to the circuit elements formed on the substrate to form a complete electronic circuit pack. Substrate heating is accomplished on a hot belt in a linear reflow soldering system sold by the Browne Corporation of Santa Barbara, Calif. under the model designation LR-6.
In the linear reflow soldering machine, a thermo-conductive belt is continuously moved over a series of heated platens, which are heated to the desired range of temperatures. The substrates carrying the separate circuit components, that are as yet not soldered to the surface of the substrates, are placed on the belt and travel over successive platens to heat the upper surface of the ceramic to the various desired soldering temperatures, in a particular sequence. The heat transfer is from the underlying platen, through the belt and through the ceramic to the solder-coated connector surfaces of the tantalum circuit elements formed on the surface of the substrate and then to the leads of the separate circuit components. Too low a temperature applied to the ceramic substrate will result in improper soldering. Too high a temperature could damage the separate components being soldered to the surface of the substrate.
In order to test the temperature profile of the upper surface of a substrate carried on the belt of the Browne Reflow Soldering Machine, at the various locations along the path of the belt, a thermocouple is held against the upper or test surface of a ceramic substrate used as a test fixture. The leads of the thermocouple are connected to a digital display unit. The substrate, with the thermocouple attached, and the digital display unit are placed on the belt and move along the belt as would a normal ceramic circuit pack being reflow soldered. The digital display unit, which stands on heat-resistant legs to protect the unit from the heat of the belt, is then read from time-to-time as the thermocouple-bearing substrate moves along the belt of the Browne Machine so that a series of time-dependent temperature readings can be obtained.
It is known to hold the thermocouple to the ceramic substrate with putty in order to attach the thermocouple to the top or tested surface of the ceramic substrate. However, this tends to give spurious readings due to the excessive insulating effect of the bonding material and, when the bonding material becomes loose, give oppositely spurious readings due to the poor contact between the thermocouple and the substrate. The bonding materials tend to have difficulty accommodating the wide temperature ranges encountered during the repeated heating and cooling cycles going from room temperature to the soldering temperature on the conveyer belt and then back to room temperature. Typically, putty would break up and fall off within a week.